Heat transfer coils find many uses today to transfer heat from one fluid to another. In instances where both fluids between which heat is to be transferred need to be confined, heat transfer coils consist generally of one or more tubes through which one of the fluids is passed with the first mentioned tube being enclosed in another tube so that the other liquid passes between the first mentioned tube or tubes and the second mentioned tube. One of the primary problems with this type of heat transfer coil construction is that, once the size of the first mentioned tube is selected, the area through which the heat is transferred from one of the fluids to the other fluid is typically fixed unless one goes to expensive fabrication techniques and uses excessive materials in order to place fins on the tubes carrying the first mentioned fluid. Another problem encountered with this type of prior art heat transfer coil is that it is difficult to form such heat transfer coils in a coil configuration in which both the first mentioned tube or tubes and the second mentioned tubes are curved since it is difficult to maintain the concentricity between the tubes resulting in a varying heat transfer efficiency between the fluids.
In some cases, such as those in which heat is to be transferred to or from potable water, safety code regulations require a double wall between the fluids in a heat transfer relationship with each other. This is the case when condenser heat from refrigeration, air conditioning or heat pump systems is used to heat potable hot water. In order to place a double wall between the refrigerant and the potable water being heated, the prior art, as best illustrated in U.S. Pat. Nos. 3,922,876 and 4,173,872 has attempted to solve this problem by sheathing the tube carrying the refrigerant in an extra tube so that three, rather than two, tubes are used in the coil. In such coils, the potable water typically passes through the innermost tube while the refrigerant passes between the outermost tube and the middle tube. The space between the innermost tube and the middle tube is typically filled with a heat conducting medium in an attempt to provide good heat transfer between the refrigerant and the potable water. This type of prior art heat transfer coil suffers from several drawbacks. One of these drawbacks is that such heat transfer coil is difficult and expensive to fabricate. Another drawback is that it is difficult to maintain a good heat transfer rate between the refrigerant and the potable water. Yet another drawback is that this type of heat transfer coil requires the use of at least three tubes to transfer heat between two fluids and, as such, uses an excessive amount of tubing material and produces a heavy coil. Still another drawback is that this type of coil requires soldered or mechanical joints at the tube ends.